Continuity mistake: During the dinner party, Philip crushes a glass in his hand, and we see that his hand is cut and bloody before he wraps a piece of cloth around it. Later on, when Mrs. Atwater reads his palms, not only is there no bandage, but there is also no blood or sign of a cut on his hand.
Continuity mistake: When David is killed in the very first scene, the two men lay him neatly in the big chest, the rope does not hang out the side. But then one man bends over and discovers the rope halfway hanging out of the side of the chest. Plus it was knotted around David's neck anyway, so couldn't have been yanked out, it would have had to be untied.
Continuity mistake: The chest holding the dead man's body is almost waist high on all the characters in all of the scenes, except the last shot when Jimmy Stewart walks toward it. The chest is now about knee height on him, and he sits down beside it and comfortably rests his arm on the chest with the gun in his hand.
Continuity mistake: Jimmy Stewart has two ice cream sundaes in his hands, and both have a dark topping (likely chocolate) as he is talking to the character Brandon. The shot changes and the toppings change to one dark and one butterscotch (light topping).